Awesome report Steve! Thank you!

Your mod job on the Euro-5 looks meticulous and very CLEAN! Awesome job!
Actually... there's another option for you to look at:
The BQC is a great circuit, which does sound great at 18v.
The Spector USA 18v Circuit has a great tone, quality and I honestly think it's my favorite circuit overall.
But recently I had an opportunity to revisit Aguilar's circuits in a new way and I have to admit I was pretty much blown away.
The bass below was made for Quintin Berry. It is a custom job done as a "thank you" to Quintin for all of his years as a loyal, faithful Spector Artist. And Stuart even gave it a special serial number, NS-4Q #1-804. It's NS-4 #804. But it's the first model made with this special control setup. And Stuart realized more people may want it.
PJ hasn't instructed me to change the NS-4 webpage yet, meaning it's not officially being offered as of yet. But I want one. I love the two compact, small knobs and how you get so much beautiful NS-Body wood with this setup.
The two knobs are actually routed into the space that the former (no longer offered) piezo bridge controls fit into.
This NS-4, when I plugged it in and dialed in my Redwood NS-4's settings... sounded just like my Water-Cured Redwood. So I immediately opened the back cover to see how Stuart had shoe-horned the USA 18v circuit into that configuration.
And I was shocked to find an OBP-2 sitting there staring back at me... running at 18v. For the next hour I A-B'd my NS-4 and Quintin's NS-4 and the sound was so close most of the time that I made it a point to talk to Dave Boonshoft at Aguilar during BPL about it.
Here's why I want to try the OBP-3 at 18v now. I didn't realize it... but Dave explained that the OBP-3 is special because they used both Operational Amplifiers (think Spector USA 9v Circuit) and JFET (think Spector USA 18v and TonePump Circuit) to make that single 3-band OBP-3 circuit.
So it's the best of both worlds... the analog, solid-state growl and grit with the clean, powerful, articulate JFET tech.
I've honestly decided to revisit the OBP-2 and OBP-3 circuits because I think when you add the extra headroom of another 9v battery to them it elevates their abilities into a totally different tonal realm.
The EMG is good... but it sounds like an EMG circuit... and I just can't get over how much the 2-band OBP-2 sounded like the 2-band Spector 18v circuit when you run the OBP-2 at 18v.
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